1. Field of the Invention
The present invention relates to a servo control system, and more particularly, to a servo control system utilizing a phase-locked technique for controlling the speed of a step motor.
2. Description of Prior Art
A step motor is an electromagnetic incremental actuator that converts input pulses to shaft rotation. Since pulse trains are digital signals, controllers for driving step motors can be combined with digital systems. Phase-locked techniques have been widely used in servo control systems of step motors. The control may be related to controls of the shaft of a step motor to rotate accurately at a desired angular speed, or related to positioning controls of an movable member driven by the step motor.
Among the known phase-looked techniques, a charge-pump phase-locked servo system employing a phase detector and a passive RC filter is disclosed in a technical paper "PHASE-LICKED LOOPS FOR MOTOR-SPEED CONTROL" proposed by Moore and published on IEEE SPECTRUM, Vol. 10, pp. 61-67, April 1973. Up to the present time, loop controllers employed in phase-locked systems are often of a charge-pump type. A charge pump controller basically consist of a phase detector (PFD) and a loop filter which can be either passive or active. During each pumping period, the pumped voltage sent out by the loop filter increases exponentially with the characteristic curve thereof much alike to that of an RC circuit. As a result, there is a non-linear relationship between the pumped voltage and the input phase error, which causes the phase-locked system to respond to the phase error variations differently in different speed ranges of the step motor.
In addition, at the instant during each pumping period when a phase error is detected, an impulse-like voltage jump always appears in the loop filter, which appears eventually as part of the output of the charge-pump controller. The impulse-like voltage jump seriously affects performances in the control of a step motor.
Furthermore, speed transducers employed for detecting the angular speed of a step motor are usually of a type which utilizes an optical encoder. The function of the optical encoder is equivalent to a frequency multiplier, i.e. a complete revolution of the step motor shaft will actuate the optical encoder to generate a predetermined number of pulses. The predetermined number is technically referred to as a line density of the optical encoder. Therefore, the angular speed of the step motor can be determined by measuring the frequency (pulses per second) of the output pulse train of the optical encoder. The larger the line density, the higher the speed resolution becomes.
It is, however, found by Margaris and Petridis that a design choice with a higher line density for the optical encoder causes the step motor speed servo controller to have a lower system stability. The discovery is described in a technical paper "PLL SPEED REGULATION OF FRACTIONAL HORSEPOWER SERIES AND UNIVERSAL MOTORS" on IEEE TRANSACTIONS ON INDUSTRIAL ELECTRON, Vol. IE-31, pp. 277-281, Aug. 1984. Accordingly, a tradeoff often has to be made between line density and system stability.